Using GROUP by does not meet the criteria he does not return data, how to solve

The group by does not meet the criteria he does not return data
Such as: posts post table
Create TABLE posts (
POSTS_ID,
ForumID, (forum ID)
Posts_title,
Posts_posttime,
)

Comments Comment Form
CREATE TABLE Comments (
COMMENTS_ID,
Postsid,
Comments_content,
Comments_posttime,
)

The result I want is: Read all posts with Forum ID 1 and count the total number of comments for each post.
I use COUNT (comments_id) as commentsnum to comment, but the post is not commented on the comment form, group by ignores
posts_id ForumID Commentsnum
1 1 4
2 1 5
3 1 4
4 1 0
I want to be a post in the comment table when no comments, return 0, or return null also OK, how to get

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------Solution--------------------

Select A.posts_id,a.forumid,count (b.comments_id) as Commentsnum  
From posts a left join comments B on A.posts_id=b.postsid
GROUP BY B.POSTSID have a.forumid=1

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